Mocking

Ensures test isolation by replacing external dependencies with controllable mock objects during test execution.

Unit testing: isolating a function that calls an external API or service Integration testing: replacing a database connection with a deterministic mock Legacy code: intercepting calls to untestable dependencies without refactoring

Unit test files using unittest.mock, test suites, test cases.

with patch('module.Class') as Alias:

with patch('module.Class') as Alias:

module_path: dotted string path to target class or function, alias: valid Python identifier for mock reference

• patch
• Mock
• assert_called
• assert_called_with
• return_value
• side_effect

• patch.object: patches an object attribute rather than a module-level name
• useful for instance attributes
• patch.dict: patches dictionary entries rather than object attributes
• @patch decorator: patches for entire test function
• less precise scoping than context manager

Failing to stop the patch, patching the wrong attribute, forgetting to assign the mock to a variable, not using autospec leading to overly permissive mocks.

patch.object (patches attribute on an object), patch.dict (patches dictionary items), unittest.mock.MagicMock (direct mock creation).

Coming from JavaScript/Jest: may expect mock hoisting behavior where jest.mock() is hoisted to file top → Python's patch must be placed at the correct import resolution point, typically where the name is looked up not where it is defined

• unittest.mock patterns

Do not use patch for dependencies you own and can refactor to accept via constructor injection — prefer explicit dependency injection for cleaner design. Patching has negligible runtime overhead but each patched test is harder to reason about than constructor-injected doubles. The patch target string must match where the name is looked up (usage site), not where it is defined, which is a common source of silent patching failures.

Indicates a testing boundary where ExternalService is replaced with a test double.

Like hiring a stunt double for a scene — the real actor steps out, the double performs exactly as directed, and when the scene ends the real actor returns.

Ensure the target string matches the import path used in the code under test.

Consider using patch.object for more precise attribute patching or autospec for stricter mocks.